Car Alarm LED modification

Discussion in 'Automotive Electronics' started by cwdusmc, May 31, 2016.

  1. cwdusmc

    Thread Starter New Member

    May 31, 2016
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    Hello all, new to the forum but I've been lurking around for quite a while trying to figure out things for myself. I'm very green at circuitry on this level but am actively self-teaching to learn more. I understand the basics of a lot of the most used components in circuit boards, but I'm stumped on my latest endeavor. I recently had a car alarm installed that has a 3V output powering a red LED but it's very dim and can't be seen during the day and barely at night - so, I grabbed a 5V bright blue LED to put in place. Of course, I knew the module wouldn't power it to the level I wanted, so I integrated a small board with a 2N2222 transistor and 350 ohm resistor on the collector side. Problem I have is that the alarm module blinks the led once every 5 seconds and it's barely a blink (split second) so it's not powering my "aftermarket" LED the way I would like. I believe including a 555 timer may help in my efforts to get my bright blue LED to "hold" the blink for at least a second, but every other time I've used a 555 timer, the control voltage had to be constant (not a pulse), not sure how to integrate (another transistor?) the timer into my circuit. To sum it up: in my ideal world, I would like to have blink my bright blue LED every 2-3 seconds from the alarm's factory slow blink 3V output. I know I just butchered my explanation of this and I don't have a schematic but hopefully it's enough to understand. Any input is greatly appreciated.
    Thanks!
    Clint
     
  2. cwdusmc

    Thread Starter New Member

    May 31, 2016
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    Here's a schematic of what I have - not sure how to make the pulsing 3V output of the alarm module trigger the transistor long enough to light my blue LED. Correction to my first post, it's actually a 330 ohm resistor. I know all this may seem trivial just for an LED, but it's what I'm doing right now. Again, thanks for any input

    Alarm LED.JPG
     
  3. #12

    Expert

    Nov 30, 2010
    16,298
    6,808
    The reason you aren't getting answers is that you asked for too much, completely changing all the timing from a tiny pulse.
    What you drew is a pulse stretcher, but it goes more like this:
    There is a way to do it with a 555 timer and get a sharp, "off" for the LED, but that's more work.
    Good enough to have a sharp, "on" then just let it fade away?
    (The 1M resistor is probably a bit high.)
     
  4. cwdusmc

    Thread Starter New Member

    May 31, 2016
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    Thanks a lot, however what is this symbol? I've not come across it yet and am not afraid to ask:

    Capture.JPG

    Sharp "on" and fade away is just fine - I'll skip the 555 for now
     
  5. #12

    Expert

    Nov 30, 2010
    16,298
    6,808
    That's an N-channel enhancement type mosfet.
    They are very good at low voltage drop in the LED circuit and very good about gate leakage current. That's why I can stick a million ohms in there and expect the gate to work.
    You need one that's good for at least 60 volts in a car.
    Personally, I would go for a 100 volt n-channel mosfet. Cars can get so hinky about voltage spikes!
    Anything on this page will do:
    http://www.mouser.com/Semiconductor...w78k9Z1yw78izZ1yw76b4Z1yw78faZ1yw764iZ1yw78f4
     
    cwdusmc likes this.
  6. cwdusmc

    Thread Starter New Member

    May 31, 2016
    4
    0
    Thanks again for your help, this is great info
     
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